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The orifice opens into the branchial cavity behind a conical lobe, which stands above the third foot in place of a branchia which is wanting in Ocypoda. It is bounded laterally by ridges, which rise above the articulation of the foot, and to which the lower margin of the carapace is applied. Exteriorly, also, it is overarched by these ridges with the exception of a narrow fissure.
From this point of view we might certainly say that a widely gaping fissure which had nothing prejudicial in it to Aratus Pisonii among the foliage of the mangrove bushes, was not suitable to the Ocypoda living in sand; that in the latter, in order to prevent the penetration of the sand, the orifice of the branchial cavity must be placed at its lowest part, directed downwards, and concealed between broad surfaces fringed with protective brushes of hair.
This fissure is overlaid by the carapace, which exactly at this part projects further downwards than elsewhere, and in this way a complete tube is formed. Whilst in Grapsus the water is allowed to reach the branchiae only from the front, I saw it in Ocypoda flow in also through the orifice just described.
In the position of posterior entrant orifice and the accompanying peculiarities of the third and fourth pairs of feet, two other non-aquatic species of the same family, which I have had the opportunity of examining, agree with Ocypoda.
In the case before us it is sufficient to refer to the Gelasimus of the mangrove swamps, which shares the same conditions of life with various Grapsoidae and yet does not agree with them, but with the arenicolous Ocypoda.
Nevertheless it may be remarked that in both species, especially in Ocypoda, the olfactory filaments in their ordinary situation are very much reduced, and when they are in the water their flagella never perform the peculiar beating movements which may be observed in other Crabs, and even in the larger Gelasimus; moreover, the organ of smell must probably be sought in these air-breathing Crabs, as in the air-breathing Vertebrata, at the entrance to the respiratory cavity.
In Eriphia gonagra the entrant orifices of the respiratory cavity serving for aerial respiration are situated, not, as in the Grapsoidae, above, but behind the last pair of feet at the sides of the abdomen. Posterior entrance to the branchial cavity of Ocypoda rhombea, Fab., natural size. The carapace and the fourth foot of the right side are removed.
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